Transcutaneous Electrical Nerve Stimulation (TENS) devices apply electrical currents to particular areas of the human body in order to suppress pain. The most common form of TENS is called conventional TENS. In a conventional TENS device, an electrical circuit generates stimulation current pulses with specified characteristics. The pulse waveform specifications include intensity (mA), duration (μsec) and shape (typically monophasic or biphasic). The pulse pattern specifications include the frequency (Hz) of the stimulation pulses and the length of each continuous stimulation session (minutes).
Electrical stimulation is typically delivered to the user through electrodes, with the electrical stimulation being in the form of low intensity (typically less than 100 mA), short duration (typically 50-400 μsec) pulses at frequencies typically between about 10 and 200 Hz. The electrodes are placed on the skin of the user within, adjacent to, or proximal to, the area of pain. The electrodes typically utilize hydrogels to create a stable low-impedance electrode-skin interface to facilitate the delivery of electrical current to the user so as to stimulate peripheral sensory nerves, whereby to suppress pain.
Poor sleep quality is one of the major causes of morbidity in patients suffering from chronic pain [Fishbain D A, Hall J, Meyers A L, Gonzales J, Mallinckrodt C. Does pain mediate the pain interference with sleep problem in chronic pain? Findings from studies for management of diabetic peripheral neuropathic pain with duloxetine. J Pain Symptom Manage. December 2008; 36(6):639-647]. It is, therefore, desirable that patients have the option of receiving TENS therapy during sleep. In fact, several studies have shown that TENS therapy can improve sleep quality (see, for example, Barbarisi M, Pace M C, Passavanti M B, et al. Pregabalin and transcutaneous electrical nerve stimulation for postherpetic neuralgia treatment. Clin J Pain. September 2010; 26(7):567-572).
A significant safety concern for traditional TENS use is the potential for “electrode peeling” (i.e., where the electrodes of the TENS device unintentionally separate from the skin of the user). Such electrode peeling can result in current density and power density increases due to decreased electrode-skin contact area. Increased current density and power density could lead to painful stimulation and, in the extreme, thermal burns. The U.S. Food and Drug Administration (FDA) has published draft guidelines on TENS devices that require a warning against the use of conventional TENS devices during sleep due to the risk of unintended electrode peeling [Food and Drug Administration, Draft Guidance for Industry and Staff: Class II Special Controls Guidance Document: Transcutaneous Electrical Nerve Stimulator for Pain Relief, Apr. 5, 2010]. Consequently, most TENS devices are designed to operate exclusively during the day (i.e., during wake state) without any nighttime (i.e., sleep state) operation.
In pending prior U.S. patent application Ser. No. 14/230,648, filed Mar. 31, 2014 by Shai Gozani et al. for DETECTING CUTANEOUS ELECTRODE PEELING USING ELECTRODE-SKIN IMPEDANCE, there is disclosed an invention which allows TENS therapy to be applied during nighttime (i.e., during sleep state) as well as during the day (i.e., wake state). In accordance with the aforementioned invention, the TENS device is adapted to measure electrode-skin impedance continuously during the TENS therapy for monitoring electrode-skin contact area. The known geometry of the pre-configured electrode array establishes the initial electrode-skin contact area and the analysis of subsequent electrode-skin impedance changes allows an accurate estimation of electrode-skin contact area (i.e., to detect the occurrence of electrode peeling). When the impedance reaches a critical value corresponding to a reduced contact area (i.e., the occurrence of electrode peeling) that may lead to excessive stimulation current density or power density, the device automatically terminates stimulation in order to avoid the risk of painful stimulation and, in the extreme, thermal burns.
To achieve maximum pain relief, electrical stimulation needs to be at an adequate intensity level [Moran F, Leonard T, Hawthorne S, et al. Hypoalgesia in response to transcutaneous electrical nerve stimulation (TENS) depends on stimulation intensity. J Pain. 12:929-935)]. The optimal TENS therapeutic intensity level is often described as the intensity level that evokes “strong but comfortable” sensation from a user. However, a stimulation intensity level tailored for daytime (i.e., wake state) use may be too strong for nighttime (i.e., sleep state) use since the stimulation intensity level appropriate for the wake state may interfere with sleep.
Another common feature of a TENS device is its “on-demand” pain relief operation. A pain-relieving TENS therapy session typically starts immediately when the user interacts with the device in a prescribed manner, such as by pressing a button on the TENS device. Each therapy session typically lasts for about 60 minutes. While on-demand therapy gives the user complete control over the timing of each TENS therapy session, on-demand therapy is not well suited for nighttime (i.e., sleep state) use as it requires deliberate and regular interaction with the TENS device by the user.
For these reasons, it would be advantageous to provide automated means to detect the user's sleep-wake state so that the TENS device can automatically alter its operation according to the daytime (i.e., wake state) or nighttime (i.e., sleep state) needs of the user. In addition to delivering effective pain therapy during the daytime, it would be advantageous if the TENS device could adjust its therapeutic stimulation parameters (such as the stimulation intensity level) during nighttime (i.e., sleep state) so as to avoid interference with sleep. Thus, in order to strengthen its utility at nighttime (i.e., during sleep state) for pain relief without interrupting sleep, the TENS device should monitor the sleep state and sleep quality of the user and adapt its operation (e.g., adjust its stimulation intensity level) accordingly.
The gold standard in determining the sleep-wake state of a subject is polysomnography which comprises at least three distinct types of data (i.e., EEG, EOG and EMG). Because of difficulty in recording and analyzing these types of data, actigraphy has been developed and refined over the last 30 years as a practical alternative to study sleep/awake patterns [Ancoli-Israel S, Cole R, Alessi C, Chambers M, Moorcroft W, Pollak C P. The role of actigraphy in the study of sleep and circadian rhythms. Sleep. May 1, 2003; 26(3):342-392]. Actigraphy is a continuous recording of body movement by means of a body-worn device that detects body movement, typically through the use of accelerometers. Significantly, the present invention integrates an actigraphy-based sleep-wake classification method with conventional TENS device functionality in order to provide a novel method and apparatus for enhancing the pain-relieving utility of TENS therapy at nighttime (i.e., during sleep state) without disturbing the sleep of the TENS user.